In the direct conversion method, IQ signals in the baseband are directly converted into RF (Radio Frequency) signals. At this time, signal quality may degrade due to the amplitude/phase error and DC offset of an analog quadrature modulator/demodulator. If the transmitter has DPD (Digital Pre-Distortion), the nonlinear distortion compensation performance of DPD may degrade due to the amplitude/phase error and DC offset of the analog quadrature modulator/demodulator. To avoid these problems, methods of estimating a quadrature error caused by the analog quadrature modulator/demodulator using digital signal processing have widely been examined.
When estimating a quadrature error, it is assumed to have no frequency dependence. However, when the EVM (Error Vector Magnitude) of the analog quadrature modulator/demodulator is actually measured, an image remaining due to a quadrature error is observed. The cause of the residual image is considered to be the frequency characteristic of the measurement system. Non-patent literature 1 introduces a method of estimating/correcting a quadrature modulator/demodulator error having frequency dependence (Frequency Dependent IQ Imbalance).
In current products, an image remains due to the frequency dependence of a quadrature error based on the skew difference between an I channel and a Q channel. The skew difference is about 100 psec, and it is difficult to suppress the skew difference by an analog circuit. Hence, the skew difference is corrected by digital signal processing. However, it is hard to measure a delay difference of 100 psec, and the MER (Modulation Error Ratio), EVM, or the like needs to be measured. For example, the ISDBT method uses a measuring device capable of measuring the MER on the carrier basis. It is possible to correct the skew difference using the frequency characteristic of the MER measured by the measuring device. However, only limited types of measuring devices can measure the frequency characteristic of the MER. Depending on the broadcast method, for example, CMMB, there exists no measuring device capable of measuring the MER on the carrier basis, and the skew difference cannot be corrected.